Porous metal bodies of uniform porosity

ABSTRACT

SINTERED POROUS METAL SHEETS SUPPORTED OR UNSUPPORTED BY A BACKING SHEET AND HAVING NOT MORE THAN ABOUT ONE PORE PER TWENTY-FIVE (25) SQUARE FEET OF SHEET WITH A SIZE LARGER THAN THREE TIMES THE AVERAGE PORE SIZE OF SAID SHEET.

Int. Cl. 322E 3/10 U.S. Cl. 29-1823 8 Claims ABSTRACT OF THE DISCLDSURESintered porous metal sheets supported or unsupported by a backing sheetand having not more than about one pore per twenty-five (25) square feetof sheet with a size larger than three times the average pore size ofsaid sheet.

This application is a division of copending application, Ser. No.798,142, filed Feb. 10, 1969, now US. Pat. 3,577,226, which in turn is adivision of application Ser. No. 650,250, filed June 30, 1967, now US.Pat. 3,433,632 which is, in turn, a continuation-in-part of applicationSer. No. 484,123 filed Aug. 31, 1965 and now abandoned.

This invention relates to porous metal bodies. More particularly theinvention is directed to porous metal sheet and to an improved processfor producing it.

A number of methods have been employed heretofore in producing porousmetal bodies, particularly porous metal sheet. These methods includesintering of metal particles, the use of materials which liberate gas atelevated temperatures in order to introduce voids into a metal product,and the use of slip casting techniques in which metal particles aresuspended in a variety of liquid or solid binders and then heated toeliminate the solvent or binder. All of these methods were subject toone or both of the disadvantages of non-uniform porosity in the productor difiiculties in continuous large scale production.

It is an object of this invention to provide an improved process forproducing porous metal bodies, including porous metal sheets,particularly very thin sheets of highly uniform porosity. A furtherobject of the invention is to provide porous metal sheets of highlyuniform porosity which can be produced in a continuous process. A stillfurther object of the invention is to provide porous metal sheets whichcomprise two or more layers of different metals or two or more layers ofthe same metal but with different porosity.

According to the process of this invention a fluid mixture is preparedwhich comprises: (1) a metal powder, (2) an organic thickening agent,(3) an organic plasticizer, and .(4) a volatile solvent. The fluid mixis then deposited in a thin layer on a backing sheet and allowed to dry,that is, the solvent is evaporated. This (substantially solvent free)material, referred to as green sheet, is then subjected to furthertreatment to vaporize the organic material and sinter the metalparticles. If desired, the sintered porous sheet can then be strippedfrom the backing sheet.

Any sinterable metal powder can be employed in this process, andsuitable powdered metals, include nickel, copper, cobalt, iron,tungsten, silver, stainless steel, other alloys such as nickel-basealloys, iron-base alloys, and cobalt-base alloys and the like. Typicalstainless steel powders include types 304 and 316 as defined in theMetals Handbook, 8th edition, pp. 408-409. Other illustrative alloypowders include (a) those having the composition (weight percent)nickel, 76%; chromium, 15.8%;

3,677,721 Patented July 18, 1972 iron, 7.2%; manganese, 0.2%; silicon,0.2%; carbon, 0.04%; and copper, 0.1% and (b) those described in US.Pat. No. 2,703,277. The metal particle size is not'critical, butgenerally particles having an average diameter in the range of 1 to 40microns are preferred. The choice of particle size depends primarily onthe desired pore size of the metal sheet, the smaller particlesproviding a product with a correspondingly smaller pore size.

Also, non-sinterable powders can be admixed with the sinterable metalpowders. The sintered powder then provides a matrix which supports thenon-sinterable material. Useful non-sinterable materials include carbonpowder and Raney nickel powder.

The organic thickening agents (that is, agents whose function is toprovide strength and durability to the green sheet and to define thestructure of the porous sheet) which are useful in the process includeplastic materials such as nitrocellulose, methyl cellulose, ethylcellulose, and the like, and other organic materials such as thealginates.

The organic plasticizers are employed inorder to improve the flexibilityand ease of handling of the green sheet, and to improve the film-formingproperties of the thickening agent. Useful plasticizers includeconventional plasticizers such as paraflin oils, dimethyl phthalate,polyoxyalkylene glycols, and the like.

The combination of organic thickening agent and plasticizer used in thisinvention differs from organic materials used as binders in prior arttechniques in that green sheet ductility and flexibility are obtainedwithout undesirable cracking pinholding, and mud cracking. Theseproperties are obtained by coating the individaul metal particles with amixture of organic materials not requiring thermal treatments.

The volatile solvents which are useful in the process of this inventioninclude water, alcohols such as methanol, isopropanol, and the like,aromatic hydrocarbons such as benzene and toluene, and ketones such asacetone. Miscible mixtures of these solvents can also be used.

An organic wetting agent can be included in the fluid mixture as anoptional ingredient. The wetting agent improves the homogeneity of themix and the uniformity of dispersion of the metal particles. Suitablewetting agents include stearic acid and quaternary ammonium salts suchas lauryl isoquinolinium bromide and dimethyl ammonium chloride.

All of the organic thickening agents, organic plasticizers, organicwetting agents, and solvents useful in this invention are materialswhich will either volatilize, completely decompose, or oxidize atelevated temperatures without leaving solid residues.

The backing sheet can be, for example, metal, porous metal, expandedmetal, glassine paper, plastic coated paper, plastic'sheet, and thelike.

The fluid mix containing the metal powder, thickening agent,plasticizer, and solvent can be prepared by any convenient method. Onesuch method is to first mix the thickening agent, plasticizer, andsolvent, and then add the metal powder and continue mixing until auniform suspension of metal powder is obtained. The relative amounts ofthe metal powder, thickening agent, plasticizer, and solvent whichresult in a uniform suspension depend upon the particular materialsemployed, and also on the particular metal and the particle size of themetal. The compositions of a number of satisfactory suspensions aregiven in the illustrative examples hereinbelow.

After the suspension of metal powder has been prepared, it can bedeposited on the backing sheet by any convenient method. In continuousprocesses, it has been found particularly convenient to deposit the mixcontaining metal powder from a standard slit feeder onto a moving sheetof flexible material (herein referred to deposited thereon.

vOf particular importance in producing the porous metal bodies of thisinvention is the ability to sinter either completely in the freeshrinkage state or completely in the restricted shrinkage state. Onlyunder properly selected conditions can these states be realized. Failureto operate under these conditions results in sheets containing pinholes,cracks, warpage, and similar defects. In the case of free shrinkage, theunsintered sheet of metal powder suspended in a plastic matrix must beperfectly free to shrink in any direction during the sinteringoperation. In the, case of the restricted shrinkage method, theunsintered sheet must be mechanically restricted, on a microscopicscale, from shrinkage appreciably in any direction except that verticalto the plane of the sheet.

It 'has been discovered that by proper selection and treatment of thebacking sheet, the proper combination of nonmetallic ingredients in themix, and the proper precompression of the unsintered sheet, thedesirable result of either free or restricted shrinkage can be obtained.The examples hereinbelow illustrate some of the combinations whichproduce defect-free porous material. Excessive prerolling, sintering onbacking strips not properly pretreated, or excessive sintering duringthe initial stages all can cause sticking of the sintered sheet to thebacking sheet precluding removal or can cause the generation ofexcessive defects in the finished porous sheet. In the case of freeshrinkage, improper mixes which do not provide sufficient coherentmaterial to bond the powder together during the initial sintering stepswill result in sheets having excessive tears, pinholes, and the likedefects.

The desirable shrinkage characteristics just described can be obtainedwhen the ingredients used in the process of this invention are mixed inthe weight ratios shown in Table I, the ratios being based on the amountof metal powder employed.

Table I Ingredient: Weight (grams) Metal powder 100 Thickening agent0.5-16 Plasticizer 0.5-9 Solvent 10-75 Wetting agent 3 These ratios alsodepend to some extent upon the powder characteristics and the desiredfinal properties of the sheet. Therefore, in the case of a specificporous metal product, the weight ratios, While falling in this range,are held to much closer tolerance. For instance, the porous productobtained from nickel powders having average diameters of about 6 to 15microns will have highly uniform properties if the ingredients used inthe process are employed in the weight ratios given in Table II.

. Table II Ingredient: Weight (grams) Nickel powder 100 Thickening agent3-5 Plasticizer 0.9-3.2 Solvent 15-20 Wetting agent Q-0.5

For smaller nickel metal powders, for example powders in the 3 to 5micron diameter range, the same mix ingredient ratios given in Table IIalso apply except that the thickening agent can be used in amounts up to8 grams per 100 grams of nickel powder and the solvent can be used inamounts up to 40 grams per 100 grams of nickel powder.

The ingredients of the metal powder containing mix, together with theother process variables referred to earlier, influence the shrinkagecharacteristics of the sheet,

and therefore, its final properties. The non-metallic ingredients in themix are added to the metal powder to provide handleability and strengthto the green sheet. Excess amounts of these ingredients cause undueshrinkage and disruption when removed. Insufficient amounts make the mixdifiicult to handle and result in a weak green sheet. The proper amountfills the void structure formed by the metal particles but does notsignificantly distend the metal structure. In this way gross shrinkageis prevented in the presintered state by virtue of the restrainingpowder particle network.

In addition to the factors described earlier with regard to free andrestricted shrinkage, the relationship of sintering conditions (time,temperature, and atmosphere) to the powder characteristics is important.These characteristics are mainly the surface area, shape, size,agglomeration, and melting point of the powder. Each of these infiuencethe degree of consolidation effected by a given set of sinteringconditions. These basic factors dictate the sintering conditions 'for agiven powder and structure; In general, the particle size of the powderand its melting point have the greatest influence on shrinkage duringsintering. Low melting points and small particle size cause rapidsintering and lead to nonuniform shrinkage unless the sinteringconditions are moderated to account for these tendencies.

The process of this invention is applicable to a wide variety of metalpowders. Some powders have very sharp particle size distributions andpack to relatively high densities; other powders have very broadparticle size distributions and are loose and fluffy in nature. However,in general, a sheet made from any of these powders by the process ofthis invention will have a maximum individual pore size less than threetimes as large as the average pore size. The actual value will bedependent upon the particular powder used and is a characteristic of thepowder and the thickness of the product sheet. In order to obtain thishigh degree of uniformity the green sheet should generally besufficiently thick to obtain metal powder particle stack ing of 15particles or more. In the porous metal products of this invention, thepore size non-uniformity (many pores larger than 3 times the averagepore size) which characterized previously known materials is practicallyeliminated; These large (non-uniform) pores will not occur morefrequently than one per 25 square feet in the porous products of thisinvention. For example, the product of Example 1 hereinbelow has anaverage pore size of 6 microns (measured by the alcohol bubble pressuremeth- 0d) and a maximum pore size of 12 microns; pores as large as 18microns occur not more than once per 25 square feet of product.

Where the metal powder is nickel powder, iron powder, or copper powder,the backing sheet is preferably preoxidized stainless steel.

After the green sheet has been deposited on the backing sheet, the finalproduct can be obtained by several different series of steps. Forexample, in one embodiment of the invention the prerolled green sheet issintered while still in contact with the backing sheet. The sinteringoperation, of course, serves to eliminate all volatile, decomposable,and/or oxidizable components as well as to effect sintering of the metalparticles. The sintered metal powder layer is then stripped from thebacking sheet and subjected to an additional sintering treatment. Arolling operation can be incorporated into the process between the twosintering treatments, if desired. In another embodiment of theinvention, the green sheet is passed through rollers while still incontact with the backing sheet, subjected to the sintering operationwhile still in contact with the backing sheet, and thereafter thesintered porous metal sheet is stripped from the backing sheet.

Where paper or plastic backing sheets are employed, the sinteringoperation decomposes and/or oxidizes the backing sheet; preferably suchbacking sheets are separated from the green sheet prior to the sinteringsteps.

The thickness of the as deposited green sheet can vary from about 0.010inch up to 0.062 inch or greater. During the evaporation of the solventthe thickness of the green sheet decreases by as much as 60 percent.

The removal of the volatile organic material and the sintering of themetal particles can be carried out in two separate steps or in a singlecombined step. To remove the volatile, decomposable and/or oxidizablematerial it is preferable to heat the green sheet slowly to at leastabout 400 C. This can be conveniently done in a stream of inert gaswhich helps to carry away the volatilized components. The sinteringoperation is then carried out in a reducing atmosphere at temperaturesof 700 C. or above, depending upon the particular metal powder. Forexample, where the metal is nickel or copper, a hydrogen atmospherecontaining some water vapor is employed; where the metal is steel oriron an atmosphere of dry hydrogen is preferred.

The removal of decomposable and/or oxidizable material and the sinteringoperations can be conveniently combined in a continuous process, forexample, by depositing the green sheet on a moving backing sheet whichis continuously passed through a furnace. The furnace temeprature andspeed of the moving sheet are adjusted so that the green sheet ismaintained at about 400 C. for about minutes and is then heated at thesintering temperature of about 700 C. to about 1000 C., depending uponthe metal used, for about 20 minutes. Cooling takes place on leaving thefurnace. The entire furnace is flushed with dry or moist hydrogen orother neutral atmosphere depending on the metal involved.

The porous metal sheets produced by this process have thicknesses fromas low as as 0.003 inch up to 0.030 inch and above. The void fraction inthe porous sheets can be as high as 60 percent and the average porediameter can vary from as low as one micron up to about 50 microns.

In an important embodiment of the present invention, porous metal sheetsare prepared which comprise two or more layers of different porousmetals or two or more layers of the same metal wherein the layers havedifferent porosity. Supported layers in such multi-layer structures canhave void fractions as high as 90 percent.

These multi-layer materials can be prepared by several proceduresJIn onemethod additional layers of mix containing metal powder can be applieddirectly to previously cast and dried green sheet. The multi-layer greensheet can then be subjected to the various rolling, sintering, annealingand stripping operations as described hereinabove.

In a second and preferred method, additional layers of mix containingmetal powder can be deposited on a finished (sintered and stripped)porous sheet and the solvent evaporated to form a green sheet on top ofthe finished sheet. The finished sheet-green sheet structure is thensintered or rolled and sintered. In this method, it is preferred to usein the second layer a metal powder which sinters at a lower temperaturethan the metal in first (finished) layer.

In a third method, separate unsintered sheets are prepared; and two ormore of these sheets are placed in contact, pressed together by passingthrough a rolling mill, and then sintered. The sintering step provides ametallurgical bond between the sheets.

The porous metal sheets produced by the procem of this invention have anumber of applications. Because of their uniformity and strength theyare excellent, high quality filters. They are uniquely suited for use aselectrodes in a wide variety of fuel cells and batteries. High porosity,large-pores structures can be made which serve as abradable seal memberstranspiration-cooling walls, and soundsuppression duct liners. Poroussheet, preferably copper or brass, can be impregnated with hearingalloys to make a high performance, long life bearing liner. The porousmetal structures can be applied to solid heat exchanger surfaces topromote nucleated boiling. Also, the combination of the backing sheetand (unstripped) porous sheet can be used in fabricating boilingpromoting surfaces in heat exchangers.

The following examples further illustrate the process and product of thepresent invention:

Example 1 The following ingredients were mixed in a conventional paintshaker; 5900 grams of acetone, 1132 grams of nitrocellulose, and 680grams of dimethylphthalate. After one hour of mixing 32 kilograms ofnickel powder (14 micron average diameter) were added and mixed for anadditional two hours. The suspension containing nickel powder had aviscosity of 2500 centipoises. This mixture was allowed to stand for 24hours to remove trapped gases, after which it was slowly rotated for twohours to rehomogenize the mix. The mixture was then loaded into aconventional slit feeder apparatus and forced by air pressure onto amoving belt of preoxidized stainless steel 0.008 inch thick and 12inches wide. The use of mild steel or copper was not successful due tothe formation of alloy between the nickel and the carrier belt whichresulted in warpage and tearing of the sintered sheet. A leveling bar infront of the slit feeder insured sheet of uniform thickness. Theleveling bar also defined the wet sheet thickness which was 0.029 inch.Upon drying the thickness reduced to 0.013 inch. The combination greensheet and stainless steel sheet was rolled to a thickness of 0.017 inch(green sheet thickness of 0.009 inch). This precompacting reducesnonuniformity, time required for sintering, and shrinkage duringsintering. If the sheet is not precornpacted, surface crackingfrequently occurs along the edge of the sheet, these cracks reduce theuniformity and the strength of the sintered sheet. The prerolled greensheet and support sheet combination was then passed through a beltfurnace. The furnace temperature and belt speed were adjusted so thatthe green sheet was heated from room temperature to 800 C. in 5 minutesand was maintained at 800 C. for 20 minutes. This tem perature of theinitial pass is limited by alloying that occurs between the green sheetand the carrier belt; in this case it can be as high as 950 C. but, inExample 2, alloying would occur at this temperature. The furnaceatmosphere was a mixture of 92.5 volume percent nitrogen and 7.5 volumepercent hydrogen which had been passed through a water bubbler. Thesintered sheet was cooled in a water-jacketed area of the furnace toabout C. before exiting from the furnace. The sintered sheet emergedcompletely separated from the stainless steel and was rolled up on aseparate pickup roll. The sintered sheet was refurnaced (resintered) inthe same atmosphere for 15 minutes at 950 C. to further reduce theporosity of the sheet. The finished sheet was 0.008 inch thick, 11inches wide, and 200 feet long.

Example 2 The following ingredients were mixed in a conventional paintshaker; 1504 grams of acetone, 311 grams of nitrocellulose, grams ofdimethylphthalate, and 70 grams of lauryl isoquinolinium bromide. Afterone hour of mixing, 5500 grams of nickel powder (4 micron averagediameter) were added and mixed for an additional hour. The mix was thenslowly rotated for 3 hours. The suspension containing nickel powder hada viscosity of about 4500 centipoise. The mix was then forced through aslit feeder onto a moving preoxidized stainless steel belt in the samemanner as described in Example 1. The green sheet had a wet thickness of0.037 inch and a dried thickness of 0.016 inch. The sheet was thenrolled as in Example 1 to a thickness of 0.008 inch, and sintered as inExample 1 with the exception that it was not necessary to resinter thismaterial. As noted in Example 1, higher temperatures during sinteringwill result in sticking to the carrier belt; this is a function of thenickel particle size.

Example 3 The following ingredients were mixed for one hour on aconventional paint shaker; 160 grams of acetone, 40 grams ofnitrocellulose, and 16 grams of dimethylphthalate. Eight hundred gramsof 7 micron nickel powder were stirred into the plastic and this mixturewas mixed for an additional hour. The mix was allowed to stand for onehour and was then cast onto a moving hard surface paper belt. The wetthickness was 0.030 inch which dried to 0.014 inch. The sheet wasallowed to dry and the green sheet and paper sheet were separated. Thegreen sheet was then rolled to 0.008 inch and sintered at 750 C. forminutes in the same manner and atmosphere as in Example 1. For this typeof operation the steps involving evolution of gases are important as thesheet is loose and weak at this point and furnacing at highertemperatures and faster rates frequently causes cracking of the greensheet. The maximum rate is dependent upon the thicknesss of the greensheet which controls its rigidity.

Example 4 A two-layer sheet was produced by casting a plastic mix onto apreviously sintered sheet as made in Example 2. It was intended that thematerial from Example 2 would provide a ductile fine-pored layer whilethe second layer would have high porosity and a large pore size. Toaccomplish the latter, the procedure described in Example 2 was followedwith the following exceptions: 2000 grams of an agglomerated 3-micronnickel powder were used; porous material from Example 2 was used as acarrier belt; and no prerolling was required. Sintering was done in thesame manner as in Example 2. The sintered sheet showed no tendency ofseparation between the layers on pressure tests. Excessive temperatureor time at temperature will cause severe curling of the edges of thesheet due to the relatively high shrinkage in the high void layer. Thevoid fraction of this layer must be controlled by prerolling orpostrolling rather than sintering.

Example 5 (A) A two-layer porous metal sheet of this invention wasproduced by the general technique of Example 4 with the exception thatthe porous metal of Example 3 was used as a carrier sheet and theplastic mix cast onto it had the following composition: 200 gramsacetone; 40 grams nitrocellulose; grams dimethylphthalate; 600 grams of4-micron nickel powder and 150 grams of minus 325 mesh Raney nickelpowder. The sintering temperature was reduced to 700 C. so that Raneynickel would not lose its activity. Other material was produced in thesame manner with the exception that it was rolled before sintering toincrease the mechanical strength of the second layer. The loading of thenickel with the Raney nickel must be such that a continuous nickelskeleton is developed since the Raney nickel is primarily physicallyheld rather than being sintered into the structure.

(B) A two-layered structure utilizing carbon instead of Raney nickel wasmade in the manner of Example 5 (A) with the exception that only 7%active carbon was used since the density of carbon is much lower. Inaddition this material was prerolled and sintered at 1000 C. for minutesto obtain maximum bonding of the nickel. The requirement of theformation of a continuous nickel skeleton as in Example 4 was thecontrolling factor in the strength of the second layer.

Example 6 As an alternate method of producing 2-layer structures,composite sheets were produced by casting material as in Example 1followed by casting material as in Example 3 directly on top of greensheet from Example I. This sheet was prerolled to reduce ditferentialshrinkage of two layers and was sintered as in Example 2.

Example 7 A sintered two-layer structure of high uniformity in bothlayers was produced by producing two separate green sheets as in Example3. The plastic-metal mix from Example l was used for casting one of thegreen sheets, and the mix described in Example 3 Was used for the secondgreen sheet. The two green sheets Were then bonded together bylaminating and rolling through a conventional rolling mill. Thelaminated sheet was then sintered as in Example 3.

Example 8 A sintered bimetal, porous two-layer structure was produced bythe general procedure outlined in Example 4 with the followingexceptions: The carrier sheet was material from Example 1 and theplastic mix consisted of 45 grams of dimethylphthalate, 40 grams ofnitrocellulose, 180 grams of acetone, and 1000 grams of 16-micron copperpowder. The sintering was accomplished in the same manner as Example 4with the exception that the sintering temperature was 950 C.

Example 9 Porous copper sheet was made using the same plasticmetal mixand sintering conditions as given in Example 8 and the casting techniqueoutlined in Example 2.

Example 10 The following ingredients were mixed in a conventional paintmixer: 1000 grams of acetone, 200 grams of nitrocellulose, and grams ofdimethylphthalate. After one hour of mixing, 4950 grams of nickel powder(7 micron average diameter) were added and mixed for an additional hour.The suspension containing nickel powder had a viscosity of about 4000centipoise. This mixture was allowed to stand for about a half hour toallow trapped vapor or air bubbles to escape. The mixture was thenloaded into a conventional slit feeder apparatus. The mix containingmetal powder was then forced by air pressure from the slit feeder onto amoving belt of preoxidized stainless steel .008 inch thick and 9 incheswide. The green sheet which had a wet thickness of 0.024 inch wasallowed to air dry for about 15 minutes. The thickness of the dry greensheet was about 0.012 inch. The combination green sheet and backingsheet was then prerolled at a thickness of 0.009 inch in order toprecompact the metal powder. The prerolled green sheet-backing sheetcombination was then passed through a furnace. The furnace temperatureand belt speed were adjusted so that the green sheet was heated fromroom temperature to 950 C. in 9.2 minutes and was maintained at 950 C.for 37 minutes. The furnace atmosphere was a mixture of 95 volumepercent nitrogen and 5 volume percent hydrogen which had been passedthrough a water bubbler. The sintered strip was cooled in a waterjacketed area of the furnace to about C. before emerging from thefurnace. The sintered porous nickel, after stripping from the backingsheet was 0.0075 inch thick, 9 inches Wide, 40 feet long and had a voidfraction of 40 to 45 percent.

Example 11 An expanded metal-supported porous stainless steel sheet wasprepared by casting a plastic mix containing stainless steel powder ontoa carrier belt of expanded metal. The film-forming characteristic of theplastic mix permitted the mix to bridge the holes in the expanded metal,and the wetting characteristics of the mix caused the plastic mix toform a thin layer of material on both sides of the expanded metal. Theplastic mix used consisted of 7500 grams of -325 mesh stainless steel(304) powder, 311 grams of nitrocellulose, 200* grams ofdimethylphthalate, 752 grams of acetone, and 752 grams of toluene. Theexpanded metal used was made from 0.005 inch thick stainless steelsheet. The strand width was 0.007 inch wide and it was pulled to a 30/0pattern roughly 0.04 inch wide diamond pattern holes. The plastic mixwas blended with the stainless steel powder by first shaking on a paintshaker for 30 minutes, then slowly rolling on a mill for 90 minutes.When it was cast, it had a viscosity of 4200 centipoises. The plasticmaterial was cast onto the expanded metal following the generalprocedures of Examples 1. The solvent evaporated rapidly and themesh-mix combination quickly became self-supporting. The green sheet wasprerolled to reduce its thickness by about percent before sintering at1200 C. for 3 minutes in a dry hydrogen atmosphere. The sheet wasuniform and well bonded to the expanded metal.

Example 12 A large-pored, high void sheet was prepared by firstpreagglomerating nickel powder, and then casting it into green sheet andsintering it so that the areas between the agglomerates formed the largepores, and the voids within the agglomerates aided in achieving a highvoid fraction. The same nickel powder used in Example 1 was presinteredat 750 C. in a hydrogen atmosphere. The presintered stock was ground upand screened into specific screen sizes of -50 to +150, 150 to +250, and-250 to +325. A mix was made using 600 grams of the 50 to +150 meshpresintered nickel powder, 200 grams of toluene, 200 grams of acetone,80 grams of nitrocellulose, and 50 grams of dimethylphthalate. The mixwas blended by shaking on a paint shaker for onehalf hour. The mix wasthen allowed to stand for one hour to remove trapped gas bubbles, andthe powder which had settled from the plastic was redispersed by handstirring. The mix was then cast on a tightly stretched nonsolubleplastic film. The material was removed from the plastic film beforesintering and was sintered at 1100 C. for 12 minutes in a dry hydrogenatmosphere. The sintered sheet was 0.04 inch thick.

Example 13 Dense bimetal strip was produced by casting the mix ofExample 4 containing 3-micron nickel powder onto a dense, mild steelcarrier belt (0.008 inch thick). The mix (after evaporation of solvent)was 0.003 inch thick. The powder was sintered and bonded to the mildsteel carrier by sintering in hydrogen at 1100 C. for 3 minutes. Thesheet was then rolled to a total thickness of 0.009 inch to provideadditional densification of the porous layer, and the two layer stripwas then annealed for 3 minutes at 1100 C. in hydrogen.

Example 14 A porous layer of copper was bonded to solid copper sheet bycasting the mix from Example 8 ontoa copper carrier sheet 0.0045 inchthick and sintering in a nitrogen atmosphere at 1025 C. for minutes. Thesintering temperature, atmosphere, and time were suflicient to provide awell bonded porous layer on the solid sheet. In addition, some of themix was applied to other shapes, primarily tubes, by dipping and slowlyrotating to provide a uniform thickness. These tubes were coated oneither the inside or outside and were sintered in a manner identical tothe flat sheet. The same degree of bonding of the porous copper layer tothe solid copper layer was obtained. Alternatively, the solid coppertubes and other shapes can be coated on both sides with the copper 10powder mix and sintered in a similar manner. These solid copper-porouscopper combinations are particularly useful in heat exchangerconstruction.

What is claimed is:

1. At least a two-layer structure comprising sintered porous metalsheets of difierent porosity wherein each sheet has no more than aboutone pore per twenty-five (25) square feet of sheet with a size largerthan three times the average pore size of said sheet and wherein themetal in each sheet is independently selected from a group consisting ofnickel, copper, cobalt, iron, tungsten, silver, stainless steel, anickel-base alloy, an iron-base alloy and a cobalt-base alloy.

2. The structure of claim 1 wherein the metal in each sheet is the same.

3. A two-layer structure comprising a backing sheet layer and a layer ofsintered porous metal sheet having not more than about one pore pertwenty-five (25 square feet of sheet with a size larger than three timesthe average pore size of said sheet and wherein the material in saidbacking sheet is selected from a group consisting of metal, stainlesssteel, glassine paper, plastic coated paper and plastic sheet, andwherein the metal in said sintered porous metal sheet is selected from agroup consisting of nickel, copper, cobalt, iron, tungsten, silver,stainless steel, a nickel-base alloy, an iron-base alloy and acobalt-base alloy.

4. The structure in accordance with claim 3 wherein said backing sheetis porous metal.

5. The structure in accordance with claim 3 wherein said backing sheetis expanded metal.

6. The structure in accordance with claim 3 wherein said backing sheetis preoxidized stainless steel and wherein the metal in said sinteredporous metal sheet is selected from a group consisting of nickel, ironand copper.

7. The structure in accordance with claim 3 wherein said backing sheetis solid copper and the metal in said sintered porous metal sheet iscopper.

8. The structure in accordance with claim 7 wherein said structure istubular in shape.

References Cited UNITED STATES PATENTS 3,397,968 -8/1968 Laucadel et al-222 X 3,535,110 10/1970 Todd 75-222 X 3,577,226 5/ 1971 Elbert et al.75-222 X 3,195,226 7/1965 Valyi 75-222 X 3,214,270 10/1965 Valyi 75-222X 3,266,893 8/ 1966 Buddy 75-222 X 3,323,879 6/ 1967 Kerstetter et al.75-222 X 3,331,684 7/1967 Storchheim 75-222 X 3,335,002 8/1967 Clarke75-22-2 X 3,336,134 8/1967 Kulp et al. 75-222 X 3,351,464 11/ 1967Budincsevits 75-222 X 3,362,818 1/ 1968 Schwarzkopf 75-222 X 3,382,0675/1968 Sandstede et al. 75-222 X 3,384,482 5/ 1968 Kelley et al. 75-222X CARL D. QUARFORTH, Primary Examiner R. E. SCI-IAFER, AssistantExaminer US. Cl. X.R. 29-1823 mum STATES PATENT owzcn QE'HFEQATE mPatent No. 3 ,677,721 Issue Date Julv 18 1972 Inventofls) R. J. Elbertat 81 It is certified that error appears in the above-identified patentand that said Letters Patent are hereby can-acted as shown below:

, w Em Column 8, line 75 "30/0" Should be --3/0-- Column 9, line 42"0.003" shouia be --o.oos-- Signed and sealed this 1st day of May 1973.

(SEAL) Attest:

EDEEARD M. FLETCHER, JR. ROBERT GOTTSCHAIK Attesting OfficerCommissioner of Patents Patent No. 3,677 721 Issue Date Julv 18 1972Inventot(s) R. J. Elbert et a1.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 8, line 75 "30/0" should be --3/0-- Column 9, line 42 I "0.003"should be --0.008-- Signed and sealed this 1st day of May 1973.

(SEAL) Attest:

EDHARD M. FLETCHER, JR. ROBERT GOT'I'SCHAIK Attesting OfficerCommissioner of Patents

